1. Field of the Invention
The invention relates to pressure swing adsorption for gas separation. More particularly, it relates to the tuning of vacuum pressure swing adsorption systems to maintain stable high performance.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes have been used to separate and purify gases in highly significant applications, such as the separation of air to produce oxygen or nitrogen product gas. Most PSA processes are carried out in multi-bed systems with each bed undergoing the same sequence of steps but in a different phase relationship with other beds in the system. Such processes involve adsorption at an upper pressure level, desorption at a lower pressure level, and repressurization from the lower pressure to the upper pressure. Many PSA processes also employ one or more pressure equalization steps or repressurization steps in which gas is withdrawn from one bed at high pressure and passed, directly or through the use of a surge tank, to another bed initially at lower pressure until the pressures in said beds are equalized. This enables some compression energy to be saved, and generally also enhances the overall recovery of product gas, e.g., oxygen, in the process,
In vacuum pressure swing adsorption (VPSA) processes, the lower desorption pressure is a sub-atmospheric, i.e., vacuum, desorption pressure. In a desirable VPSA processing sequence for air separation, the following sequence of operating steps is carried out, on a cyclic basis, in one or more adsorbent beds capable of selectively adsorbing a more adsorbable component of a feed gas mixture from the less adsorbable component thereof; (1) feed gas pressurization of each adsorbent bed from an intermediate pressure to the upper, super-atmospheric adsorption pressure; (2) feed gas introduction to the feed end of the bed at said upper adsorption pressure, with adsorption of the more adsorbable component and simultaneous withdrawal of the less adsorbable component from the product end of the bed; (3) countercurrent depressurization to a lower pressure, with release of gas from the feed end of the bed; (4) evacuation to a lower sub-atmospheric, i.e., vacuum, desorption pressure; (5) optional purge, typically by the introduction of a small amount of product gas to the product end of the bed at the lower desorption pressure; and (6) repressurization of the bed to the intermediate pressure level. This VPSA processing sequence can be used in a single bed system or in multi-bed systems containing two or more adsorbent beds. In variations of such processing, a co-current depressurization step or steps can be employed in which gas is released from the product end of the bed, as for pressure equalization to an intermediate pressure with another bed in the system.
The PSA process, including the VPSA process, is a transient process that is influenced by external disturbances and variables. In some cases, once a disturbance is introduced to the system, it does not automatically self-correct itself in response to such changes. Instead, the problem may grow, and self perpetuate until the PSA process can no longer operate at peak capacity or efficiency. Variables that can affect the PSA process include outside ambient temperature, inlet feed conditions, process equipment variability, process valve positioning and response time, and a variety of other factors. In order for the PSA process to run optimally, it must be monitored in order to determine if an outside variable has had an impact on the process. Once this has been determined, steps can be taken to correct and compensate the system in order to force it to run in its optimal condition once again. One variable in a PSA process that has been found to have a great impact on the performance of the system is adsorbent vessel temperature profiles. Typically, in a VPSA process, especially with advanced high capacity adsorbents, the temperature profile in the axial direction follows these trends:
Bottom Adsorbent Vessel Temperature (Inlet Feed Gas End):
30.degree. to 60.degree. F. below the Feed Gas Temperature
Middle Adsorbent Vessel Temperature (at a Point Halfway between the Inlet Feed Gas End and the Product Outlet End of the Vessel):
10.degree. to 30.degree. F. below the Feed Gas Temperature.
Top Adsorbent Vessel Temperature (Product End of the Vessel):
Within .+-.10.degree. F. of the Feed Gas Temperature.
In a PSA process, if the adsorbent vessel temperature profile follows the above trends, then the process is assumed to be running in a stable "tuned" condition. However, a small disturbance in the process can very easily skew the vessel temperature profiles, causing them to be very different from the criteria indicated above. From experience, it can be seen that when the adsorbent vessels have very different temperature profiles from those normally observed in the art, then product recovery will decrease, unit power consumption will increase, and overall plant performance will be lower. This problem becomes even more critical as newer and better performing adsorbent materials are used in PSA, including VPSA, systems.
In order for a VPSA system to perform optimally, it has heretofore been necessary to manually adjust the pressure equalization or repressurization and purge flows for each adsorbent vessel in the system to assure that each adsorbent bed was achieving the production of similar product purities. This has typically been done by connecting an oxygen analyzer to a point downstream of the product withdrawal point to the beds and measuring the breakthrough purities of the product gas. Once the breakthrough purities were balanced, the process was said to be "tuned", and maximum product gas capacity would be produced at the lowest unit power consumption.
In the Abel et al. U.S. Pat. No. 4,995,089, the control of product flow from an adsorption separation system when the downstream customer has a discontinuous use pattern is addressed. A differential pressure controller is used to measure the differential pressure (DP) in a product pipeline. On the basis of the DP value measured in the line, the differential pressure controller sends an appropriate pneumatic signal to a valve in the pipeline in order to control product flow. This automatic control approach is related solely to the control of pipeline product flow to a customer, and is not directed to the actual PSA process operation, or to optimizations that can make the process more efficient.
In another PSA control approach, Schebler et al., U.S. Pat. No. 4,589,436, disclose the use of a small bleed valve in conjunction with an oxygen partial pressure monitor in order to control the partial pressure of oxygen in the product stream. If said partial pressure were to rise above a certain preset limit, the bleed valve could open, thus allowing a small portion of product gas to escape into the atmosphere. This causes the PSA plant to produce more gas, which, in turn, lowers the oxygen purity in the product stream, thus lowering the oxygen partial pressure in this stream. While this patent discusses the control of oxygen partial pressures by increasing product gas flow, it does not relate to the lowering of product flow in an efficient manner, so as to avoid an increase in unit power consumption, and does not relate to PSA vessel temperature, or the use thereof to control product purity from a specific PSA vessel in response to the process disturbances referred to above.
Another automatic control for a PSA system is disclosed in the Gunderson patent, U.S. Pat. No. 4,725,293. In order to control impurity levels in the product stream, the inlet feed flows are changed in response to the purity levels in the product stream. However, the compression machinery desirably used to provide a feed stream to the PSA system are constant displacement machines, and the actual volumetric inlet feed gas, e.g., air, is relatively constant, with cycle times being altered in order to change the total amount of feed gas used in a PSA process. In the practice of the approach recited in the patent, a reduction in the feed gas, with other processing variables kept constant, will result in a corresponding reduction in product flow. In addition, the patent makes no mention of the monitoring of adsorbent bed vessel temperatures in order to control product purity in the course of PSA processing operations.
The use of PSA systems to supply oxygen under variable demand conditions is disclosed in the Grader patent, U.S. Pat. No. 4,643,743. At maximum design capacity of the PSA process, the oxygen product/feed air ratio is a set value. As oxygen demand level from a customer falls from design flow conditions, the oxygen product feed air ratio is increased pursuant to the patented process. The overall product purity level decreases, but the actual oxygen content level in the product stream passing to a wastewater treatment operation is maintained at the desired flow level. Thus, the feed air flow is decreased, or the product flow rate is increased, thereby reducing the overall oxygen purity, but supplying the correct oxygen flow to the customer under reduced demand conditions. In the practice of other PSA applications, it is desired to maintain the oxygen purity at a constant level. The patent provides no teaching with respect to bed temperature monitoring in order to produce constant purity from each adsorbent bed in a PSA processing operation.
The Miller et al. patent, U.S. Pat. No. 4,693,730, discloses a method for controlling the purity of a gas component in a PSA product stream. Co-current depressurization, i.e., pressure equalization, gas is analyzed to determine whether a product purity problem exists. Once this is determined, then action can be taken to restore the purity level of the product stream to its correct level. The patent suggests three approaches to correct a purity problem existing in the process. Thus, the adsorption step time can be adjusted in order to control impurity loading in each adsorbent vessel of a PSA system; the final depressurization step pressure can be adjusted to avoid impurity breakthrough; or the amount of purge gas entering each adsorbent vessel can be adjusted during the purge step. Such actions are taken in response to the monitoring of the pressure equalization gas purity level. The patent does not suggest bed temperature monitoring, nor efficiencies in power consumption in turning down product flow from a PSA system when a customer does not draw maximum design flow rates from the process.
The method of tuning a PSA system by the Miller et al. approach requires sampling of the oxygen approach breakthrough purities of the adsorbent beds in the system. This requires the use of an additional oxygen analyzer and an onsite adjustment of the process controls in order to "tune" the PSA system and balance the product flow and purities between the adsorbent beds. This procedure requires several iterations over a relatively long period of time, typically about 12-24 hours.
There is a need in the art for an improved method of tuning PSA systems, including VPSA systems. While controlling and changing aspects of the product stream in order to optimize product purity in the VPSA system has thus been suggested, the approach of tuning adsorbent vessels, in order to improve process performance, has not been disclosed in the art. The advantage of an approach directed to the tuning of adsorbent vessels would be that such an approach would attempt to compensate for any process inefficiencies before they are able to negatively impact PSA plant performance. In the cases of the prior art approaches, the monitoring of the product stream is to determine whether there is a process problem that needs to be corrected. At this point in time, product flow and/or purity has already been affected, and must be remedied. The solving of potential process problems before they occur is inherently a more advantageous way in which to control the PSA, e.g. VPSA process.
It is an object of the invention to provide a process for tuning PSA, including VPSA, systems for maintaining stable, high performance operation.
It is another object of the invention to provide a VPSA vessel tuning process based on monitoring adsorbent vessel temperature profiles during the course of VPSA operations.
It is a further object of the invention to provide a VPSA tuning process automatically compensating for the adverse effects of processing disturbances and enabling optimum operating conditions to be maintained.
With these and other objects in mind, the invention is hereafter described in detail, the novel features thereof being particularly pointed out in the appended claims.